feat(gpu): add support for GPU-accelerated expand on the HL Api

- includes documentation about GPU's accelerated expand on the HL API
- rework CudaKeySwitchingKey
- Cloning the key is no longer necessary on the HL API

Author: pdroalves

Makefile

@@ -294,7 +294,7 @@ check_typos: install_typos_checker
 .PHONY: clippy_gpu # Run clippy lints on tfhe with "gpu" enabled
 clippy_gpu: install_rs_check_toolchain
 	RUSTFLAGS="$(RUSTFLAGS)" cargo "$(CARGO_RS_CHECK_TOOLCHAIN)" clippy \
-		--features=boolean,shortint,integer,internal-keycache,gpu,pbs-stats,extended-types \
+		--features=boolean,shortint,integer,internal-keycache,gpu,pbs-stats,extended-types,zk-pok \
 		--all-targets \
 		-p $(TFHE_SPEC) -- --no-deps -D warnings
 
@@ -892,7 +892,7 @@ test_high_level_api: install_rs_build_toolchain
 
 test_high_level_api_gpu: install_rs_build_toolchain install_cargo_nextest
 	RUSTFLAGS="$(RUSTFLAGS)" cargo $(CARGO_RS_BUILD_TOOLCHAIN) nextest run --cargo-profile $(CARGO_PROFILE) \
-		--features=integer,internal-keycache,gpu -p $(TFHE_SPEC) \
+		--features=integer,internal-keycache,gpu,zk-pok -p $(TFHE_SPEC) \
 		-E "test(/high_level_api::.*gpu.*/)"
 
 test_high_level_api_hpu: install_rs_build_toolchain install_cargo_nextest
@@ -1066,7 +1066,7 @@ check_compile_tests: install_rs_build_toolchain
 .PHONY: check_compile_tests_benches_gpu # Build tests in debug without running them
 check_compile_tests_benches_gpu: install_rs_build_toolchain
 	RUSTFLAGS="$(RUSTFLAGS)" cargo $(CARGO_RS_BUILD_TOOLCHAIN) test --no-run \
-		--features=experimental,boolean,shortint,integer,internal-keycache,gpu \
+		--features=experimental,boolean,shortint,integer,internal-keycache,gpu,zk-pok \
 		-p $(TFHE_SPEC)
 	mkdir -p "$(TFHECUDA_BUILD)" && \
 		cd "$(TFHECUDA_BUILD)" && \

tfhe-benchmark/benches/integer/zk_pke.rs

@@ -418,11 +418,11 @@ fn cpu_pke_zk_verify(c: &mut Criterion, results_file: &Path) {
 #[cfg(all(feature = "gpu", feature = "zk-pok"))]
 mod cuda {
     use super::*;
-    use benchmark::utilities::{cuda_local_keys, cuda_local_streams};
+    use benchmark::utilities::cuda_local_streams;
     use criterion::BatchSize;
     use itertools::Itertools;
     use tfhe::core_crypto::gpu::{get_number_of_gpus, CudaStreams};
-    use tfhe::integer::gpu::key_switching_key::CudaKeySwitchingKey;
+    use tfhe::integer::gpu::key_switching_key::{CudaKeySwitchingKey, CudaKeySwitchingKeyMaterial};
     use tfhe::integer::gpu::zk::CudaProvenCompactCiphertextList;
     use tfhe::integer::gpu::CudaServerKey;
     use tfhe::integer::CompressedServerKey;
@@ -451,14 +451,17 @@ mod cuda {
             let param_name = param_name.as_str();
             let cks = ClientKey::new(param_fhe);
             let compressed_server_key = CompressedServerKey::new_radix_compressed_server_key(&cks);
+            let sk = compressed_server_key.decompress();
             let gpu_sks = CudaServerKey::decompress_from_cpu(&compressed_server_key, &streams);
+
             let compact_private_key = CompactPrivateKey::new(param_pke);
             let pk = CompactPublicKey::new(&compact_private_key);
-            let d_ksk = CudaKeySwitchingKey::new(
-                (&compact_private_key, None),
-                (&cks, &gpu_sks),
-                param_ksk,
-                &streams,
+            let ksk = KeySwitchingKey::new((&compact_private_key, None), (&cks, &sk), param_ksk);
+            let d_ksk_material =
+                CudaKeySwitchingKeyMaterial::from_key_switching_key(&ksk, &streams);
+            let d_ksk = CudaKeySwitchingKey::from_cuda_key_switching_key_material(
+                &d_ksk_material,
+                &gpu_sks,
             );
 
             // We have a use case with 320 bits of metadata
@@ -609,7 +612,6 @@ mod cuda {
                             });
                         }
                         BenchmarkType::Throughput => {
-                            let gpu_sks_vec = cuda_local_keys(&cks);
                             let gpu_count = get_number_of_gpus() as usize;
 
                             let elements = zk_throughput_num_elements();
@@ -637,20 +639,17 @@ mod cuda {
                                 .collect::<Vec<_>>();
 
                             let local_streams = cuda_local_streams(num_block, elements as usize);
-                            let d_ksk_vec = gpu_sks_vec
+                            let d_ksk_material_vec = local_streams
                                 .par_iter()
-                                .zip(local_streams.par_iter())
-                                .map(|(gpu_sks, local_stream)| {
-                                    CudaKeySwitchingKey::new(
-                                        (&compact_private_key, None),
-                                        (&cks, gpu_sks),
-                                        param_ksk,
+                                .map(|local_stream| {
+                                    CudaKeySwitchingKeyMaterial::from_key_switching_key(
+                                        &ksk,
                                         local_stream,
                                     )
                                 })
                                 .collect::<Vec<_>>();
 
-                            assert_eq!(d_ksk_vec.len(), gpu_count);
+                            assert_eq!(d_ksk_material_vec.len(), gpu_count);
 
                             bench_group.bench_function(&bench_id_verify, |b| {
                                 b.iter(|| {
@@ -673,14 +672,16 @@ mod cuda {
                                         (gpu_cts, local_streams)
                                     };
 
-                                b.iter_batched(setup_encrypted_values, |(gpu_cts, local_streams)| {
-                                    gpu_cts.par_iter()
-                                        .zip(local_streams.par_iter())
-                                        .enumerate()
-                                        .for_each(|(i, (gpu_ct, local_stream))| {
-                                            gpu_ct
-                                                .expand_without_verification(&d_ksk_vec[i % gpu_count], local_stream)
-                                                .unwrap();
+                                b.iter_batched(setup_encrypted_values,
+                                               |(gpu_cts, local_streams)| {
+                                               gpu_cts.par_iter().zip(local_streams.par_iter()).enumerate().for_each
+                                               (|(i, (gpu_ct, local_stream))| {
+                                                   let d_ksk =
+                CudaKeySwitchingKey::from_cuda_key_switching_key_material(&d_ksk_material_vec[i % gpu_count], &gpu_sks);
+
+                                        gpu_ct
+                                            .expand_without_verification(&d_ksk, local_stream)
+                                            .unwrap();
                                     });
                                 }, BatchSize::SmallInput);
                             });
@@ -698,16 +699,15 @@ mod cuda {
                                         (gpu_cts, local_streams)
                                     };
 
-                                b.iter_batched(setup_encrypted_values, |(gpu_cts, local_streams)| {
-                                       gpu_cts
-                                           .par_iter()
-                                           .zip(local_streams.par_iter())
-                                           .for_each(|(gpu_ct, local_stream)| {
-                                               gpu_ct
-                                                   .verify_and_expand(
-                                                       &crs, &pk, &metadata, &d_ksk, local_stream
-                                                   )
-                                                   .unwrap();
+                                b.iter_batched(setup_encrypted_values,
+                                               |(gpu_cts, local_streams)| {
+                                               gpu_cts.par_iter().zip(local_streams.par_iter()).for_each
+                                               (|(gpu_ct, local_stream)| {
+                                        gpu_ct
+                                            .verify_and_expand(
+                                                &crs, &pk, &metadata, &d_ksk, local_stream,
+                                            )
+                                            .unwrap();
                                     });
                                 }, BatchSize::SmallInput);
                             });

tfhe/docs/configuration/gpu_acceleration/zk-pok.md

@@ -0,0 +1,82 @@
+# Zero-knowledge proofs
+
+Zero-knowledge proofs (ZK) are a powerful tool to assert that the encryption of a message is correct, as discussed in [advanced features](../../fhe-computation/advanced-features/zk-pok.md).
+However, computation is not possible on the type of ciphertexts it produces (i.e. `ProvenCompactCiphertextList`). This document explains how to use the GPU to accelerate the
+preprocessing step needed to convert ciphertexts formatted for ZK to ciphertexts in the right format for computation purposes on GPU. This 
+operation is called "expansion".
+
+## Proven compact ciphertext list
+
+A proven compact list of ciphertexts can be seen as a compacted collection of ciphertexts which encryption can be verified.
+This verification is currently only supported on the CPU, but the expansion can be accelerated using the GPU.
+This way, verification and expansion can be performed in parallel, efficiently using all the available computational resources.
+
+## Supported types
+Encrypted messages can be integers (like FheUint64) or booleans. The GPU backend does not currently support encrypted strings.
+
+{% hint style="info" %}
+You can enable this feature using the flag: `--features=zk-pok,gpu` when building **TFHE-rs**.
+{% endhint %}
+
+
+## Example
+
+The following example shows how a client can encrypt and prove a ciphertext, and how a server can verify the proof, preprocess the ciphertext and run a computation on it on GPU:
+
+```rust
+use rand::random;
+use tfhe::CompressedServerKey;
+use tfhe::prelude::*;
+use tfhe::set_server_key;
+use tfhe::zk::{CompactPkeCrs, ZkComputeLoad};
+
+pub fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let params = tfhe::shortint::parameters::PARAM_GPU_MULTI_BIT_GROUP_4_MESSAGE_2_CARRY_2_KS_PBS_TUNIFORM_2M128;
+    // Indicate which parameters to use for the Compact Public Key encryption
+    let cpk_params = tfhe::shortint::parameters::PARAM_PKE_MESSAGE_2_CARRY_2_KS_PBS_TUNIFORM_2M128;
+    // And parameters allowing to keyswitch/cast to the computation parameters.
+    let casting_params = tfhe::shortint::parameters::PARAM_KEYSWITCH_MESSAGE_2_CARRY_2_KS_PBS_TUNIFORM_2M128;
+    // Enable the dedicated parameters on the config
+    let config = tfhe::ConfigBuilder::with_custom_parameters(params)
+        .use_dedicated_compact_public_key_parameters((cpk_params, casting_params)).build();
+
+    // The CRS should be generated in an offline phase then shared to all clients and the server
+    let crs = CompactPkeCrs::from_config(config, 64).unwrap();
+
+    // Then use TFHE-rs as usual
+    let client_key = tfhe::ClientKey::generate(config);
+    let compressed_server_key = CompressedServerKey::new(&client_key);
+    let gpu_server_key = compressed_server_key.decompress_to_gpu();
+
+    let public_key = tfhe::CompactPublicKey::try_new(&client_key).unwrap();
+    // This can be left empty, but if provided allows to tie the proof to arbitrary data
+    let metadata = [b'T', b'F', b'H', b'E', b'-', b'r', b's'];
+
+    let clear_a = random::<u64>();
+    let clear_b = random::<u64>();
+
+    let proven_compact_list = tfhe::ProvenCompactCiphertextList::builder(&public_key)
+        .push(clear_a)
+        .push(clear_b)
+        .build_with_proof_packed(&crs, &metadata, ZkComputeLoad::Verify)?;
+
+    // Server side
+    let result = {
+        set_server_key(gpu_server_key);
+
+        // Verify the ciphertexts
+        let expander =
+            proven_compact_list.verify_and_expand(&crs, &public_key, &metadata)?;
+        let a: tfhe::FheUint64 = expander.get(0)?.unwrap();
+        let b: tfhe::FheUint64 = expander.get(1)?.unwrap();
+
+        a + b
+    };
+
+    // Back on the client side
+    let a_plus_b: u64 = result.decrypt(&client_key);
+    assert_eq!(a_plus_b, clear_a.wrapping_add(clear_b));
+
+    Ok(())
+}
+```

tfhe/src/core_crypto/gpu/entities/lwe_ciphertext_list.rs

@@ -7,7 +7,7 @@ use crate::core_crypto::prelude::{
 use tfhe_cuda_backend::cuda_bind::cuda_memcpy_async_gpu_to_gpu;
 
 /// A structure representing a vector of LWE ciphertexts with 64 bits of precision on the GPU.
-#[derive(Debug)]
+#[derive(Clone, Debug)]
 pub struct CudaLweCiphertextList<T: UnsignedInteger>(pub(crate) CudaLweList<T>);
 
 #[allow(dead_code)]

tfhe/src/core_crypto/gpu/entities/lwe_compact_ciphertext_list.rs

@@ -10,6 +10,10 @@ use crate::core_crypto::prelude::{
 pub struct CudaLweCompactCiphertextList<T: UnsignedInteger>(pub CudaLweList<T>);
 
 impl<T: UnsignedInteger> CudaLweCompactCiphertextList<T> {
+    pub fn duplicate(&self, streams: &CudaStreams) -> Self {
+        Self(self.0.duplicate(streams))
+    }
+
     pub fn from_lwe_compact_ciphertext_list<C: Container<Element = T>>(
         h_ct: &LweCompactCiphertextList<C>,
         streams: &CudaStreams,

tfhe/src/core_crypto/gpu/entities/lwe_keyswitch_key.rs

@@ -10,6 +10,7 @@ use crate::core_crypto::prelude::{
     UnsignedInteger,
 };
 
+#[derive(Clone)]
 #[allow(dead_code)]
 pub struct CudaLweKeyswitchKey<T: UnsignedInteger> {
     pub(crate) d_vec: CudaVec<T>,

tfhe/src/core_crypto/gpu/mod.rs

@@ -993,7 +993,7 @@ pub unsafe fn fourier_transform_backward_as_torus_f128_async<T: UnsignedInteger>
     );
 }
 
-#[derive(Debug)]
+#[derive(Clone, Debug)]
 pub struct CudaLweList<T: UnsignedInteger> {
     // Pointer to GPU data
     pub d_vec: CudaVec<T>,
@@ -1005,6 +1005,17 @@ pub struct CudaLweList<T: UnsignedInteger> {
     pub ciphertext_modulus: CiphertextModulus<T>,
 }
 
+impl<T: UnsignedInteger> CudaLweList<T> {
+    pub fn duplicate(&self, streams: &CudaStreams) -> Self {
+        Self {
+            d_vec: self.d_vec.duplicate(streams),
+            lwe_ciphertext_count: self.lwe_ciphertext_count,
+            lwe_dimension: self.lwe_dimension,
+            ciphertext_modulus: self.ciphertext_modulus,
+        }
+    }
+}
+
 #[derive(Debug, Clone)]
 pub struct CudaGlweList<T: UnsignedInteger> {
     // Pointer to GPU data

tfhe/src/high_level_api/backward_compatibility/compact_list.rs

@@ -3,6 +3,9 @@ use tfhe_versionable::{Upgrade, Version, VersionsDispatch};
 
 use crate::{CompactCiphertextList, Tag};
 
+#[cfg(feature = "zk-pok")]
+use crate::ProvenCompactCiphertextList;
+
 #[derive(Version)]
 pub struct CompactCiphertextListV0(crate::integer::ciphertext::CompactCiphertextList);
 
@@ -17,9 +20,6 @@ impl Upgrade<CompactCiphertextList> for CompactCiphertextListV0 {
     }
 }
 
-#[cfg(feature = "zk-pok")]
-use crate::ProvenCompactCiphertextList;
-
 #[derive(VersionsDispatch)]
 pub enum CompactCiphertextListVersions {
     V0(CompactCiphertextListV0),